Wound closure involves the migration of epithelial and subcutaneous tissue adjacent the wound towards the center of the wound until the wound closes. Unfortunately, closure is difficult with large wounds or wounds that have become infected. In such wounds, a zone of stasis (i.e. an area in which localized swelling of tissue restricts the flow of blood to the tissues) forms near the surface of the wound. Without sufficient blood flow, the epithelial and subcutaneous tissues surrounding the wound not only receive diminished oxygen and nutrients, but, are also less able to successfully fight microbial infection and, thus, are less able to close naturally. Such wounds have presented difficulties to medical personnel for many years.
For example, skin ulcers are a common problem among many diabetics, and are often brought on by poor blood circulation and nerve damage associated with diabetes and/or vascular disease. The treatment of such ulcers often involves grafting skin from a relatively healthy donor site to an ulcerous wound site. Split thickness surgical skin graft techniques may be employed to obtain skin grafts from donor sites that can then heal spontaneously. Full thickness skin grafts, on the other hand, generally require closure of the donor site. Furthermore, many wounds can become stalled in a “chronic condition” in which further healing does not occur and, in fact, wound may actually increase in size and depth.
Wound dressings have been used in the medical industry to protect and/or facilitate healing of open wounds. Although various types of dressing materials have been successfully employed, membranes comprising semi-permeable materials are often preferred because they can increase patient comfort and lower the risk of infection. Semi-permeable membranes generally pass moisture vapors, but are generally impervious to liquids. Thus, they can promote healing by permitting a wound site to “breathe”. An industry standard is Tegaderm™ sold by 3M Company, St. Paul, Minn. Although transparent dressings can “breathe”, they often do not have sufficient moisture vapor transmission rates (MVTR) to allow evaporation of excess wound fluid exudate. If allowed to accumulate and/or remain over the wound optimal wound healing will not occur.
In surgical wounds this is alleviated by using a wound drain that removes excess fluid to a remote container using an applied vacuum (reduced pressure). Use of wound drains often uses a separate incision to introduce the drain. Many wound dressings for chronic wounds absorb excess wound fluid. Examples include hydrocolloid adhesive dressings, absorbent foam dressings, alginate dressings, hydrogel dressings and the like. While these dressings absorb excess wound fluid they can become saturated and allow wound fluid to build up in highly exuding wounds. Further they will not optimize the wound healing environment for wounds that tend to remain dry. These dry wounds may be characterized by insufficient blood flow to the wound bed.
Another technique has been to use negative pressure therapy, which is also known as suction or vacuum therapy. These devices apply a vacuum to a wound bed beneath a film dressing. In addition to removing excess wound fluid, the vacuum is believed to allow flow of interstitial fluid into the wound bed to promote healing. Commercial devices are sold by KCI under the “Wound Vac” tradename and by Smith and Nephew (formerly Bluesky Medical) under the tradename “VISTA”. These devices comprise a motorized electric vacuum pump, wound dressing and a wound fluid trap. The wound pumps are reusable so to minimize contamination of other patients a fluid trap is placed between the vacuum pump and the wound dressing. Thus, when the trap is filled the therapy must be interrupted to change the trap. Finally, since the entire system operates at reduced pressure it becomes difficult, if not impossible, to remove a wound fluid sample for analysis without interrupting therapy. These systems are generally large and may not be easily portable.
Smaller systems have been suggested such as in U.S. Patent Application Publication No. 2007/0078366 A1 which discloses a composite wound dressing apparatus consisting of a multilayer wound dressing and a micropump. The wound dressing is described as having a base layer, a packing layer, an absorbent layer, and a top sheet. The top sheet is said to be sealed to seal the wound dressing (paragraph 0032). Paragraph 0034 states that “The micropump 120 may be embedded within the absorbent layer 106 or mounted to the layer 106, or alternatively associated within the confines of the wound dressing 100.” Thus, during operation the micropump is sealed within the cavity formed by the wound dressing and the wound, as illustrated in FIGS. 1, 2, 4 and 6 of U.S. Patent Application Publication No. 2007/0078366. The micropump is said to pull a vacuum on the wound bed (see e.g. paragraph 0034). This appears fundamentally impossible with the arrangement disclosed. Because the micropump is located within a sealed cavity having no exit from the dressing, a vacuum cannot be generated without exhausting fluid (air or liquid) from the wound cavity. As described and illustrated the inlet and outlet of the micropump are both within the wound cavity compartment.
A further problem with the composite dressing design disclosed in U.S. Patent Application Publication No. 2007/0078366 is that many (or perhaps most) wounds that require vacuum therapy can generate large volumes of fluid. The disclosure provides that removal of fluid from the dressing occurs by opening an access door (see paragraph 0033) and removing the saturated absorbent layer. For many wounds this could require frequent changes which is inconvenient, unnecessarily exposes the healthcare worker to body fluids, and requires significantly more labor than current systems which collect the exudate into a canister.
Ease of use, efficiency of healing a wound, and a source of constant negative pressure are ongoing issues that need to be addressed by continuing improvements in wound therapy.